Most of the flowmeters or simply meters in existence at present include moving mechanical parts. This is especially true of spinner and membrane flowmeters.
In contrast, fluidic oscillators do not have any moving part which could wear out with time, and therefore oscillators do not have to be recalibrated.
Such oscillators can be of small size and of very simple structure. Their reliability is thus very high. Moreover, they provide a frequency signal which can easily be converted into a digital signal. This characteristic is particularly advantageous for remote reading of meters.
Most of the efforts to develop such flowmeters have been applied to vortex flowmeters commonly called vortex effect flowmeters and Coanda effect flowmeters.
The principle of operation of vortex effect flowmeters is based on the well known fact that the presence of an obstacle in a duct in which a fluid is flowing gives rise to periodic release of vortices. The measurement principle consists in detecting the frequency of detachment of the vortices, which is proportional to the rate of flow, for an obstacle of given geometry.
The frequency of the vortices is measured in various ways, allowing the mean rate of release and thus of the flow to be determined. Vortex effect flowmeters are generally very sensitive to noise and to the upstream fluid conditions. In practice a flow rectifier is used to render the speed profile uniform. A flowmeter of this type is described in the patent U.S. Pat. No. 3,589,185 for example.
The Coanda effect used in the flowmeters of the same name consists in the natural tendency of a fluid jet to follow the contours of a wall when the jet is discharged near to this wall, even when the contour of the wall diverges from the discharge axis of the jet. A fluidic oscillator of this type comprises a chamber in which the fluid jet discharges through a convergent nozzle. Two lateral walls are located in the chamber, symmetrically about to the discharge axis of the jet. The jet issuing from the input to the oscillator attaches itself spontaneously to one of the lateral walls through the Coanda effect. A portion of the flow is then diverted by a side channel of the wall to which the jet has attached, which results in the jet detaching from this wall and attaching itself to the opposite wall. The phenomenon repeats itself and leads to a permanent oscillation of the input flow. Unfortunately the range of flow measurement is relatively limited with this type of apparatus and the non-linearity of the calibration curve is rather large. Moreover, this type of apparatus can cease to oscillate under certain conditions involving external perturbations, and a loss of signal results. In order to increase the possible range of measurement, Okabayushi et al. have proposed in U.S. Pat. No. 4,610,162 to combine two fluidic oscillators, one operating at low flow rates and the other at high flow rates.
Because of the problems encountered with vortex effect and Coanda effect flowmeters, attempts have been made to develop other types of fluidic oscillators which operate in accordance with fundamentally different principles. One application is to be found in the flowmeters described in U.S. Pat. Nos. 4,184,636, 4,244,230 and 4,843,889.
For example, U.S. Pat. No. 4,244,230 describes a fluidic oscillator flowmeter located in a duct in the path of the fluid, of which it takes off a part. The oscillator has two members located side by side with face to face walls forming a nozzle. An obstacle has a front cavity facing the nozzle.
The cavity has a common inlet and outlet. The jet leaving the nozzle penetrates into the cavity and strikes the bottom of the cavity.
The transverse oscillation of the jet in the cavity is accompanied by the formation of two vortices, one on each side of the jet. Each vortex is alternately strong and weak, each out of phase with the other. The jet leaves by the common outlet and is directed into the main flow.
Pressure sensors allow the frequency of the oscillations of the jet in the cavity to be measured, which frequency is proportional to flow rate.
The performance of this type of flowmeter is generally better than that obtained with conventional fluid flowmeters. Unfortunately the performance is not entirely satisfactory, in particular in relation to sensitivity and measurement range and also in relation to the linearity of the device in said range.
It is an object of the present invention to overcome these problems. The invention provides a fluidic oscillator and a flowmeter comprising such an oscillator of performance that is improved over that of the prior art flowmeters.
Conventionally, the linearity of such a fluidic oscillator is estimated by the relative variations in the factor K equal to the ratio of the frequency f of the oscillation of the jet divided by the flow rate Q.
In general three flow conditions are distinguished in order of increasing flow rates: laminar conditions, transition conditions, and turbulent conditions.